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Fault under Lake Tahoe tied to tsunami threat

By Carrie Peyton DahlbergSacramento Bee

Posted:
12/25/2006 08:40:09 AM PST

Updated:
12/25/2006 08:40:10 AM PST

SACRAMENTO - The biggest fault beneath Lake Tahoe could be due to rupture any time, according to an evaluation being prepared by researchers who probed Tahoe and nearby Fallen Leaf Lake earlier this year.

The preliminary conclusions, outlined earlier this month at a meeting of the American Geophysical Union in San Francisco, help sharpen a still blurry picture of potentially tsunami-spawning faults that lurk beneath the lake.

Ultimately, the findings will make their way into federal earthquake hazard maps that help determine building codes and set insurance rates.

"We have been looking at Tahoe as one of the biggest changes for California" in new maps due out in late 2007, said Mark Petersen, chief of the national seismic hazard project at the U.S. Geological Survey in Colorado.

What's come clearer with the latest Tahoe expeditions, supported by a University of California-Davis research vessel and led by scientists from three universities, are both the size and the potential of the West Tahoe fault.

The fault, which skirts the lake's west shore, runs all the way through Fallen Leaf Lake and beyond to the south, said Graham Kent, a research geophysicist at the University of California-San Diego's Scripps Institution of Oceanography.

While that didn't surprise Kent, it confirms that the fault is Tahoe's "800-pound gorilla," long enough to deliver a big jolt, magnitude 7 or greater earthquake.

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Such a quake could trigger an underwater landslide that quickly displaces huge amounts of water, potentially sending giant waves surging into parks, campgrounds, homes and marinas along the lake's shore, and possibly overtopping a dam that regulates flow into the Truckee River.

In addition, it appears from sediment layers that the last big earthquake on the West Tahoe fault was 4,000 to 6,000 years ago. That's significant, Kent said, because the fault seems to produce a major quake every 5,000 to 7,000 years, or perhaps a little more often.

That means another big earthquake could come soon - although "soon" in geologic terms could be anytime from while you're reading this to a few lifetimes from now.

Kent and Robert Karlin, a geology and geophysics professor at the University of Nevada-Reno, stressed that core samples and other data collected this summer and fall in the two lakes are still being analyzed.

More lake expeditions and three or four years of work will be needed to provide "a more definitive answer" about just how serious Tahoe's quake risk is, Karlin said.

Still, the latest assessments join a growing body of evidence about faults and landslides in the Tahoe basin, and their potential to spawn waves that could surge up to 30 feet and slosh from shore to shore for hours.

"We're keeping an eye on it," said Michael Reichle, chief seismologist at the California Geological Survey. "There are active faults near the lake, under the lake and to the east in Nevada at the base of the hills. We still don't know very much about all those faults."